1. Field of the Invention
The invention relates to fixed-ratio traction roller transmissions in which the contact forces applied to the traction surfaces which are in engagement with each other for the transmission of movement are dependent on the torque transmitted through the transmission.
2. Description of the Prior Art
Traction roller transmissions in which large contact forces are applied to prevent slippage of the rollers are described for example by Harold A. Rothbart in "Mechanical Design And Systems" Handbook pages 14-8 and 14-9, McGraw-Hill, New York 1964. In the relatively simple arrangements of FIGS. 14.6 and 14.7 wherein the outer rings are slightly undersized to compress the roller arrangements the surface pressure on the traction surfaces is always the same that is, it is always high independently of the torque transmitted through the transmission. Various transmission arrangements are also known in which the contact pressure of the traction surfaces is dependent on the size of the torque transmitted through the transmission for example as in those shown on page 14-8 of said handbook. Other transmissions of this type are shown in U.S. Pat. Nos. 771,541; 1,212,462; 1,704,205; 1,956,934, 3,610,060 and Swedish Patent No. 281,542.
In U.S. Pat. No. 771,541 the variable compressive forces are obtained by an outer ring structure so arranged that the ring is compressed when a torque is transmitted through the transmission. However, the arrangement requires sliding surface areas which are difficult to manufacture or complicated lever arrangements which amplify the reaction forces of the transmission to provide sufficient compression of the outer ring for its engagement with the traction rollers. Furthermore, there is a discontinuity in the outer ring structure which, with the large forces taken up by the ring structure, may well lead to premature failures.
U.S. Pat. No. 3,610,060 shows a friction drive in which the sun roller consists of two conical sections adapted to be forced toward each other when a torque is transmitted through the drive in order to provide firm engagement of the planetary rollers with the sun roller and the traction ring surrounding the traction rollers. However, the differences in diameter between the smallest diameter section of the sun roller and its largest diameter is relatively large so that substantial spin takes place which results in wear and friction losses. On the other hand, this transmission requires a relatively large cone angle of the conical sun roller sections in order to avoid their locking and it requires relatively long lines of contact between the sun roller sections and the planetary rollers in order to avoid overly large surface pressures which are very localized as a result of the large curvature difference between the sun roller and planetary roller traction surfaces.